Catalytic Activity For Oxygen Reduction Reaction Research Articles

Hydrogel-Derived Co3ZnC/Co Nanoparticles with Heterojunctions Supported on N-Doped Porous Carbon and Carbon Nanotubes for the Highly Efficient Oxygen Reduction Reaction in Zn-Air Batteries.

It is crucial for metal-air batteries and fuel cells to design non-precious-metal catalysts instead of platinum-based materials to boost the sluggish oxygen reduction reaction (ORR). Herein, Co3ZnC/Co nanoparticles with heterojunctions supported on N-doped porous carbon and carbon nanotubes (CNTs) are fabricated by pyrolyzing the hydrogel prepared from melamine and citric acid chelated with Co2+/Zn2+ ions. This hybrid shows strong ORR catalytic activity as its half-wave potential reaches 0.88 V (vs reversible hydrogen electrode (RHE)) in 0.1 M KOH and Zn-air batteries with the catalyst have higher discharge plateaus and capacity than those employing Pt/C. The hybrid mixed with RuO2 can also be used as an efficient bifunctional catalyst for rechargeable Zn-air batteries. The excellent performance is primarily derived from the Co3ZnC/Co heterojunctions, the electron transfer of which boosts the ORR catalysis. Moreover, the suitable ratio of Co/Zn in precursors results in the epitaxial growth of hollow CNTs and abundant mesopores, hence promoting the adsorption of oxygen and the transport of ORR-related species.

Redox Behavior and ORR Catalytic Activity of Fe-N-C Electrocatalysts in Various Electrolytes and in the Presence of Probe Molecules

Materials in the Fe-N-C family are the most promising platinum group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs). Significant progress has been made in recent years in improving both the ORR activity and durability of the Fe-N-C catalysts. However, further improvements are needed, especially in long-term performance durability in hydrogen-air PEFCs, to enable use of these catalysts in practical applications, such as powering light and heavy-duty vehicles. One of the major factors that hinder the further enhancement of the catalysts is the lack of clear understanding of the nature of the active sites in the Fe-N-C catalysts that catalyze the ORR reaction, due to the complexity of the Fe-N-C catalysts composition and structure. The most active Fe-N-C catalysts have been synthesized by pyrolyzing iron salts or other iron-containing compounds with zinc-based zeolitic imidazolate frameworks (ZIFs) and/or phenanthroline (as carbon and nitrogen sources), or by heat treating iron-substituted ZIFs. It has been shown that many synthesis variables, such as type of iron precursors, the metal and carbon-nitrogen macrocycle ratio, the heat treatment temperature, atmosphere, and temperature profile, all affect the properties and structure and thus the activity and stability/durability of the resulting catalysts. In order to further improve the Fe-N-C catalysts, it is critical to gain insights into the origins of ORR activity and the reaction mechanism. In this work, the redox behavior of different Fe-N-C catalysts, synthesized by a variety of techniques mentioned above, and correlation of this redox behavior with ORR catalytic activity in different electrolytes will be discussed and the effect of nitric oxide gas-phase adsorption on the catalyst redox feature and ORR activity will be presented. The implication of these results on the identity or identities of the ORR active sites of the Fe-N-C catalysts will be discussed.This work was supported by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO) under the auspices of the Electrocatalysis Consortium (ElectroCat 2.0). This work utilized the resources of the Advanced Photon Source, a U.S. DOE Office of Science user facility operated by Argonne National Laboratory for DOE Office and was authored by Argonne, a U.S. Department of Energy (DOE) Office of Science laboratory operated for DOE by UChicago Argonne, LLC under contract no. DE-AC02-06CH11357.

(Digital Presentation) ORR Activities on Hydrophobic Phosphonium Ionic Liquid Modified Pt/C Catalysts

PEFCs have been attracted as one of the clean and efficient energy conversion systems. Platinum nanoparticles are commonly used as cathode catalysts in Polymer electrolyte fuel cells (PEFCs) due to their high activity for oxygen reduction reaction (ORR). However, platinum is rare and expensive, therefore it needs to improve the mass activity and durability for widespread fuel cells. Recently, ionic liquids (ILs) have been studied to enhance the ORR activity of platinum catalysts1). We try to improve the catalytic activity of Pt/C catalysts by using Ionic Liquids (ILs). RTILs are salts that melt at temperatures below 100 oC and generally consist of bulky cations and anions. It has been studied in a wide range of fields, including the energy sector because of its highly conductive and nonvolatile. Recently, it has been reported that ILs layer enhance the ORR catalytic activity on Pt surface, due to the suppress the coverage of OHad from water molecules1). However, the correlation between increased ORR activity and the structure of the ionic liquid has not been clarified. In this study, we focused on ILs quaternary phosphonium cations and a bis(trifluoromethylsulfonyl)amide (TFSA) anion. The phosphonium cations based ILs have been reported to have higher thermal stability than imidazolium or ammonium cations2). In addition, the phosphonium ILs show higher electrochemical stability, conductivity, and hydrophobicity than the ammonium ILs. The ORR activity was analyzed using the rotating disk electrode (RDE) method3). Commercially available 20 wt% Pt/C (Cabot, Vulcan XC-72R\U0001f12c) catalyst was used for bare Pt/C modified electrodes. The Pt/C electrodes were modified with phosphonium ILs (PXXXY+TFSA- [alkyl chain X= 4 Y= 1, 12, 16]). The thickness of the ILs layer was calculated by using the BET surface area of carbon supports. The electrochemical measurements were conducted under Ar or O2 saturated conditions, respectively. Electrochemical surface area (ECSA) was estimated from hydrogen adsorption wave in cyclic voltammograms (CVs). Mass activity (MA) and surface area specific activity (SA) of ORR at 0.9 V on Pt nanoparticles were calculated from the Koutecky-Levich equation. The ILs layer enhanced ORR activity. However, ORR activity decreased when the ILs layer was excessively thick. These results suggest that the ORR activity decreases in a thicker ILs layer due to increased resistance to oxygen mass transport or proton conductivity. Moreover, the ORR activity of the ILs modified Pt/C catalyst increased as the asymmetric part alkyl chains of the ILs became longer. Our previous reports show that as the size -is larger, both the molar volume and oxygen solubility increases. Our results show that these physicochemical properties originating from the structure of the cation affect ORR activities.[Reference]1) Gui-Rong Zhang, et al., ACS Catal., 8, 8244-8254 (2018).2) K. Tsunashima and M. Sugiya, Electrochem. Commun., 9, 2353, (2007).3) H. A. Gasteiger, S. S. Kocha, B. Sompalli, F. T. Wagner, Appl. Catal. B Environ. 56, 9. (2005).

Strategies to improve the catalytic activity and stability of bioinspired Cu molecular catalysts for the ORR

The oxygen reduction reaction (ORR) is essential for energy conversion devices such as fuel cells and metal-air batteries. The use of expensive and scarce noble metal materials to catalyze the ORR is a limitation for the massification of these energy conversion technologies. Copper molecular catalysts that mimic the active sites of metalloenzymes such as laccase are under continuous development. In this minireview, we present the strategies to increase the activity and stability of the copper catalysts for the ORR. The flexibility, lability, and electronic character of the ligands are crucial to promote the ORR. In addition, the use of polymers as backbone for multicopper catalysts and the synthesis of copper carbon-based pyrolyzed catalysts present remarkable results with promissory applications. • The rational design of bioinspired Cu-catalysts should focus on the ligand structure. • Ligands' flexibility and lability in Cu-catalysts are crucial to promoting the ORR. • Multicopper catalysts present higher ORR catalytic activity than mononuclear ones. • The stability of multicopper catalysts should be improved. • Multicopper polymeric or pyrolyzed catalysts show remarkable ORR catalytic activity.

ZIF-8-derived Cu, N co-doped carbon as a bifunctional cathode catalyst for enhanced performance of microbial fuel cell

The development of bifunctional catalysts is an effective way to simultaneously address the slow kinetics of oxygen reduction reaction (ORR) on the cathode and biofilm contamination in the microbial fuel cells (MFC). Cu-N/C@Cu composites were synthesized as bifunctional cathode catalysts for MFC by doping, adsorption, and two calcinations by using Cu-ZIF-8 as the precursor. The higher Cu-Nx content confers excellent ORR catalytic activity to the optimized Cu-N/C@Cu-2 catalyst. The half-wave potential for Cu-N/C@Cu-2 in a neutral solution is 0.67 V vs. RHE, which is close to that of commercial 20% Pt/C (0.70 V vs. RHE). The maximum power density of the MFCs assembled with Cu-N/C@Cu-2 reached 581 ± 13 mW m−2, which is even better than that using Pt/C (499 ± 13 mW m−2). Moreover, the results of antimicrobial activity and biomass test show that the higher Cu content made Cu-N/C@Cu-2 effective against the contamination of cathode biofilm. And the 16S rDNA results find that the community structure of the biofilm is favorable for the power production and purification of MFC. This work shows that copper-based materials can be used as potential bifunctional catalysts to promote MFC applications in wastewater treatment.

Transition metal atoms anchored on square graphyne as multifunctional electrocatalysts: A computational investigation

Multifunctional electrocatalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are vital to develop energy conversion and storage devices. In this work, by means of density functional theory (DFT) computations, we have investigated the HER, OER and ORR catalytic activities of a series of transition atoms (including Sc, Ti, Cr, Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Os, Ir and Pt) anchored on square graphyne (TM@S-GPY). Our results revealed that the Ni@S-GPY can act as a promising trifunctional catalyst for HER/OER/ORR with overpotentials of 0.07/0.43/0.36 V, while Pt@S-GPY and Ir@S-GPY can be utilized as eligible bifunctional electrocatalysts for water splitting (HER/OER) with overpotentials of 0.04/0.59 and 0.07/0.36 V, respectively. In addition, Pd@S-GPY exhibits high catalytic activity for OER/ORR with overpotentials of 0.40/0.34 V. Especially, depending on their diverse electronic properties, the catalytic activity of these SACs can be well rationalized by their optimal binding strengths with reaction intermediates. Thus, S-GPY is an ideal support for immobilizing single atom catalysts to design stable and highly-efficient multifunctional catalysts in various electrochemical processes.

Exploring of catalytic oxygen reduction reaction activity of lattice carbons of vanadium and niobium doped nitrogen codoped carbon nanotubes by density functional theory

AbstractThe oxygen electroreduction mechanism on the V‐ and Nb‐doped nitrogen‐codoped (6,6)armchair carbon nanotube with incorporated MN4 fragment has been studied using the ωB97XD and PBE density functional theory approaches. The metal center in MN4 fragment and the adjacent NCCN double bond (C2 site) of the support have been revealed as active centers. The metal active centers turned out to be irreversibly oxidized at the first step of ORR affording stable O*, 2O*, or O*HO* adsorbates depending on the applied electrode potential U, that makes them no longer active in ORR. Therefore, the C2 site comes at the forefront in ORR catalysis. Among the metal oxidized forms M(O)N4, M(O)(O)N4, and M(O)(OH)N4CNT, the C2 site of the latter turned out to be most active for 4e dissociative ORR. For both metals the last protonation/electron transfer step, HO* + H* → H2O, is the rate‐limiting step. The alternative hydrogen peroxide formation is not only thermodynamically less favorable but also kinetically slower than the 4e dissociative ORR route on the C2 site of model M(O)(OH)N4CNT catalyst.

Etching-directed nitrogen doping strategy to construct efficient defective sites in carbon for electrocatalytic oxygen reduction

Construction of synergistic coupling sites of intrinsic carbon defects and N dopants in N-doped carbon is of great significance to improve the catalytic activities of metal-free carbon-based materials toward oxygen reduction reaction (ORR). However, the carbon-defect construction and N doping by existing strategies are usually independent processes, resulting in a random arrangement of the intrinsic carbon defects and N-dopants (i.e., hardly forming the coupling sites) in the resultant carbon materials. Herein, an etching-directed nitrogen doping strategy for constructing carbon defect/N dopant coupling sites is developed, in which nitrogen is directionally doped at the freshly created intrinsic carbon defects during the synthesis of N-doped carbon. The as-prepared N-doped carbon catalyst enriched with such coupling sites renders a remarkable ORR catalytic activity with an ORR half-wave potential of 0.891 V vs. RHE and an excellent long-term durability (3% current loss after 15,000 s at 0.6 V vs. RHE). Experimental characterizations and theoretical calculations reveal the formation mechanism of the coupling sites as well as the origin of their high catalytic activities toward ORR. This etching-directed nitrogen doping strategy shows great universality for the controllable construction of active sites in carbon-based materials derived from different precursors for electrochemical energy storage and conversion applications.

Robust and highly conductive Ti4O7/MXene nanocomposites as high-performance and long cyclic stability oxygen reduction electrocatalysts

Electrocatalysts play an important role in the cathodic Oxygen Reduction Reaction (ORR) of fuel cells. Because of the high cost, poor cycle stability and methanol tolerance, the commercial Pt/C catalysts have been seriously restricted during large-scale application and development in electrochemical devices. The development of non-noble metal ORR catalysts is one of the effective ways to achieve fuel cell cost reduction and performance improvement at the same time. In this work, Ti4O7/MXene nanocomposites were synthesized through hydrothermal followed by carbothermal reduction reactions. Such catalysts exhibit large electrochemical active surface area (ECSA = 215.74 cm2) and excellent ORR catalytic activity in alkaline media. The initial potential and half-wave potential were confirmed to be 0.91 V and 0.72 V with the electron transfer number of 3.3. In addition, the Ti4O7/MXene catalysts have better cyclic stability and methanol tolerance than commercial Pt/C catalyst. This study provides a new idea and method for the innovative preparation of MXene-based composite electrocatalysts.

Carbon nanotubes anchored onto hollow carbon for efficient oxygen reduction

The rational design and facile construction of non-noble metal-based electrocatalysts for efficient oxygen reduction reaction (ORR) remain a challenge for the development of clean energy conversion devices. Herein, we synthesized a new type of hierarchical hollow carbon nanomaterials through one-step pyrolysis on bimetallic ZIF-Zn/Co precursors. The obtained hollow N-doped carbon composites, denoted as HNCT-CNTs, are enriched with Co-Nx sites and in-situ formed multi-walled carbon nanotubes (CNTs). Theoretical and experimental studies confirmed that the obtained HNCT-CNT exhibits excellent ORR catalytic activity with a reduced energy barrier for ORR intermediates. Owing to these advantages, the optimal catalyst shows an excellent half-wave potential of 0.85 V, a limiting current density up to 6.36 mA cm−2, a Tafel slope of 58.2 mV dec−1, and robust stability. Furthermore, the assembled Zn-air battery based on HNCT-CNT also displays an open circuit voltage of 1.49 V and a satisfactory power density of 116.56 mW cm−2. This template-directed preparation method opens up an interesting and efficient route to fabricate highly active non-noble metal-doped carbon nanomaterials for a wide range of electrochemical energy applications.

2D RhTe Monolayer: A highly efficient electrocatalyst for oxygen reduction reaction

Oxygen reduction reaction (ORR) electrocatalysts with excellent activity and high selectivity toward the efficient four-electron (4e) pathway are very important for the wide application of fuel cells and are worth searching vigorously. In this study, r-RhTe monolayer is identified as a good ORR electrocatalyst from three 2D RhTe configurations with low Rh-loading (i.e., r-RhTe, o-RhTe and h-RhTe) on the basis of the first-principles calculations. For the most energetically stable r-RhTe, two adjacent positively charged Te atoms on the material surface can provide an active site for oxygen dissociation. Coupled with its high stability and intrinsic conductivity, 2D r-RhTe monolayer is confirmed to possess good catalytic activity and high reaction selectivity toward ORR. Moreover, under the ligand effect caused by the substitution of Cr, Mn and Fe, the ORR catalytic activity of r-RhTe monolayer could be effectively enhanced, where very small over-potential was achieved, and even comparable to or lower than the state-of-the-art Pt (111). This shows it has considerably high ORR activity. This work is highly anticipated to provide excellent candidate materials for ORR catalysis, and the related researches based on the Rh-Te materials will provide a new way to design high-performance ORR electrocatalysts to substitute the precious metal Pt-based catalysts.

Rational design of the first and second coordination spheres for copper single-atom catalyst to boost highly efficient oxygen reduction

Copper is an essential nutrient for all living things and as the component of more than 30 enzymes in the human body for the development of connective tissue, nerve coverings. However, carbon materials doped with nitrogen and transition metal singlet (MNC) with CuN4 site as reaction center usually exhibit unsatisfactory oxygen reduction reaction (ORR) activity, due to poor O2 adsorption and activation. In this work, density functional theory (DFT) was used to explore the effect of introducing heteroatoms such as B, N, P, and S into the first and second coordination spheres to improve the ORR catalytic activity for CuNC. Through the study of 41 model systems, we predicted CuN3 (ηORR = 0.32 V), CuN4P (II) (ηORR = 0.49 V) and CuN4S2(II) (ηORR = 0.37 V) fragments would have good ORR catalytic activity. This is due to the increased charge and tailored d-band fine structure, optimizing the reaction intermediate binding strength. These findings indicate the influence of the ligand field (formed by the anion in the first and second coordination spheres) on the catalytic activity and provide a new direction for the synthesis of high-efficiency electrocatalyst materials.

N-Doped CrS2 Monolayer as a Highly-Efficient Catalyst for Oxygen Reduction Reaction: A Computational Study.

Searching for low-cost and highly-efficient oxygen reduction reaction (ORR) catalysts is crucial to the large-scale application of fuel cells. Herein, by means of density functional theory (DFT) computations, we proposed a new class of ORR catalysts by doping the CrS2 monolayer with non-metal atoms (X@CrS2, X = B, C, N, O, Si, P, Cl, As, Se, and Br). Our results revealed that most of the X@CrS2 candidates exhibit negative formation energy and large binding energy, thus ensuring their high stability and offering great promise for experimental synthesis. Moreover, based on the computed free energy profiles, we predicted that N@CrS2 exhibits the best ORR catalytic activity among all considered candidates due to its lowest overpotential (0.41 V), which is even lower than that of the state-of-the-art Pt catalyst (0.45 V). Remarkably, the excellent catalytic performance of N@CrS2 for ORR can be ascribed to its optimal binding strength with the oxygenated intermediates, according to the computed linear scaling relationships and volcano plot, which can be well verified by the analysis of the p-band center as well as the charge transfer between oxygenated species and catalysts. Therefore, by carefully modulating the incorporated non-metal dopants, the CrS2 monolayer can be utilized as a promising ORR catalyst, which may offer a new strategy to further develop eligible electrocatalysts in fuel cells.

A Simple Preheating-Pyrolysis Strategy Leading to Superior Oxygen Reduction Reaction Activity in Fe-N/Carbon Black.

Maintaining the high catalytic activity of the oxygen reduction reaction (ORR) while reducing costs is a long-standing effort to promote the application of polymer electrolyte fuel cells. Here, the binding of nitrogen-containing ligands and carbon black is enhanced by controlling the pyrolysis conditions of a FeSO4 , 1,10-phenanthroline (phen), carbon black mixture, which significantly improves the ORR catalytic activity of the pyrolysis products. Preheating is proposed as a process improvement method using a heat treatment at a temperature between the melting and boiling points of phen before high-temperature pyrolysis, which achieves an effective combination of phen and carbon black, and enhances the interaction between phen and ferrous ions. This method substantially increases the number of FeNx active centers in the pyrolysis product, resulting in an impressive Fe-N/C catalyst with half-wave potential (E1/2 ) up to 0.93V and a diffusion-limited current density (jL ) of 5.9mAcm-2 and no obvious decay after 20000 cyclic voltammetry cycles in 0.1mKOH, which are all among the best-reported data known to date. The interaction between the ratio of Fe/phen and the pyrolysis conditions is also investigated. Under the right conditions, cheap raw materials can also generate highly catalytically active sites.

High catalytic activity of MBenes-supported single atom catalysts for oxygen reduction and oxygen evolution reaction

Exploring earth-abundant and inexpensive electrocatalysts to realize efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is urgently desired for developing new clean and renewable energy storage and conversion technologies. In this work, through first-principles calculations, we have studied the ORR and OER catalytic activity of the single-atom catalysts (SACs) based on different two-dimensional (2D) MBenes substrates (marked as TM-M2B2O2, TM = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn and Pt, M = Ti, Mo and W). Significantly, Ni-Ti2B2O2, Cu-Ti2B2O2, Ni-Mo2B2O2, Ni-W2B2O2 and Pt-W2O2B2 exhibit promising ORR catalytic activity with low overpotential of 0.47 V, 0.57 V, 0.23 V, 0.43 V and 0.30 V, respectively, comparable to that of Pt (0.45 V). Meanwhile, Fe-Ti2B2O2, Co-Ti2B2O2, Fe-Mo2B2O2, Co-Mo2B2O2, Ni-Mo2B2O2, Fe-W2B2O2 and Co-W2B2O2 are promising OER catalysts with low overpotential of 0.51 V, 0.46 V, 0.48 V, 0.41 V, 0.54 V, 0.58 V and 0.30 V, respectively, comparable to that of RuO2 (0.42 V) and IrO2 (0.56 V). More importantly, ICOHP was used as a descriptor of ORR and OER catalytic activity in our work and shows good guidance for the further design of ORR, OER catalysts with high catalytic activity.

Synthesis of Mesoporous Fe/N/C Electrocatalyst for Improved Oxygen Reduction Reaction Activity Through CO2‐Assisted Pyrolysis

AbstractFe/Nx/C electrocatalysts have appeared as one of the promising non‐platinum group metal (non‐PGM) electrocatalysts for oxygen reduction reaction (ORR) in energy conversion devices. These non‐PGM electrocatalysts Fe/N/C need structural tuning of pores to improve the surface area for high‐performance ORR in proton exchange membrane fuel cells. In this report, the specific surface area of poly‐melamine‐formaldehyde (PMF)‐based Fe/N/C electrocatalysts was enhanced without using a template through simple CO2‐assisted pyrolysis. The highly porous electrocatalyst exhibited an onion‐like graphitic structure and an efficient catalytic activity for ORR, comparable to Pt/C electrocatalyst. At 0.95 VRHE, the mass activity of the PMF‐based PMF−Fe/N/C‐CO2/Ar electrocatalyst was 63 % that of a commercial Pt/C electrocatalyst. Moreover, the devised fabrication process for the PMF−Fe/N/C electrocatalysts with a high specific surface area can be readily scaled up.

Porous biochar aerogel loaded PtFe alloys derived from natural marine polysaccharide for efficient oxygen reduction

Lowering the Pt loading of Pt-based materials is a promising strategy to explore excellent oxygen reduction reaction (ORR) electrocatalysts. One surprising approach is to develop Pt-alloys, which can effectively reduce the usage of Pt and induced highly catalytic activity through regulation effect between multimetals. Herein, the N-doped porous biochar aerogel/reduced graphene oxide loaded PtFe alloys (PtFe/NC-rGO (Pt1Fe5)) with low Pt loading (with a Pt content of 7.54 wt%) were prepared using natural marine polysaccharide-alginate as precursor through simple pyrolysis process. The biochar aerogel as an ideal carbon support shows an excellent electrical conductivity as well as a high specific surface area utilized for mass transport together with ion diffusion during ORR. Consequently, the PtFe/NC-rGO (Pt1Fe5) exhibits highly excellent ORR catalytic activity with respect to the pure Pt/NC-rGO together with the commercial Pt/C catalysts, with good half-wave potentials of 0.81 and 0.87 V vs. RHE in 0.5 M H2SO4 as well as 0.1 M KOH solutions, respectively. The strategy gives a new way to construct Pt-based alloy ORR electrocatalysts with low Pt consumption and the easy availability of biomass raw materials for PtFe/NC-rGO ensures its potential for large-scale application.

Thermal Migration Promotes the Formation of Manganese and Nitrogen Doped Polyhedral Surface for Boosted Oxygen Reduction Electrocatalysis.

Increasing the oxygen reduction reaction (ORR) catalytic activity of carbon-based electrocatalysts with robust stability is of great significance for their application. Herein, a feasible thermal migration strategy was proposed to construct manganese- and nitrogen-doped carbonaceous polyhedron frameworks coupled with manganese monoxide microrods (MnO-NC). Mn species were migrated to the surface of polyhedron frameworks, the shape of which was maintained at the high-temperature treatment. The Mn thermal migration not only created highly dispersed Mn-Nx active sites but also promoted graphitization, which benefited ORR electrocatalysis. Moreover, the MnO microrod-supported polyhedron frameworks provide beneficial mass transfer channels for electrocatalysis. Therefore, MnO-NC exhibited impressive ORR catalytic activity and stability in both alkaline and neutral electrolytes compared to commercial Pt/C catalysts. A magnesium-air battery (MAB) driven by MnO-NC delivered a high open circuit voltage and peak power density comparable to that driven by Pt/C. Notably, MnO-NC-driven MAB possessed a longer discharge time than the Pt/C-driven one, indicative of the superior catalytic performance of Mn-NC. This work provides a simple but effective strategy to construct carbonaceous framework electrocatalysts for boosted ORR, promoting the widespread application of metal-air batteries and fuel cells.

N-doped graphene nanoribbons intertwined on 3D graphene skeleton as superior metal-free electrocatalyst for oxygen reduction

Increasing the active sites of graphene by forming edge-rich graphene nanoribbons (GNRs) represents a promising pathway for improving the behaviors of metal-free electrocatalyst. N-doped GNRs intertwined on 3D graphene skeleton with aerogel structure (N-GNRs/G) is synthesized by hydrothermal method and subsequent pyrolysis process. In the N-GNRs/G aerogel, 3D graphene acts as the skeleton frame to construct a 3D porous structure, while GNRs with irregular edges expose abundant edge defects which offer doping sites for heteroatoms. Thus, the synergetic effect of GNRs and 3D graphene endows the synthesized N-GNRs/G with tremendous pyridinic-N and graphitic-N active sites towards the oxygen reduction reaction (ORR). The N-GNRs/G hybrid exhibits excellent ORR catalytic activity, superior stability, and selectivity. The onset potential and half-wave potential can reach 0.98 V and 0.856 V, respectively, which are close to those of the commercial Pt/C. This work paves a pathway to modulate the surface charge redistribution by means of morphology controlling and heteroatom doping for efficient and low-cost electrocatalysts.

Machine-learning-assisted discovery of highly efficient high-entropy alloy catalysts for the oxygen reduction reaction

SummaryHigh-entropy alloys (HEAs) have recently been applied in the field of heterogeneous catalysis benefiting from vast chemical space. However, huge chemical space also brings extreme challenges for the comprehensive study of HEAs by traditional trial-and-error experiments. Therefore, the machine learning (ML) method is presented to investigate the oxygen reduction reaction (ORR) catalytic activity of millions of reactive sites on HEA surfaces. The well-performed ML model is constructed based on the gradient boosting regression (GBR) algorithm with high accuracy, generalizability, and simplicity. In-depth analysis of the results demonstrates that adsorption energy is a mixture of the individual contributions of coordinated metal atoms near the reactive site. An efficient strategy is proposed to further boost the ORR catalytic activity of promising HEA catalysts by optimizing the HEA surface structure, which recommends a highly efficient HEA catalyst of Ir48Pt74Ru30Rh30Ag74. Our work offers a guide to the rational design and nanostructure synthesis of HEA catalysts.